Ok, it looks like it has worked 
I bought one of these from a hobby flying website in UK.
Installed BLHeli_32
Tried a spare ESC. The install of BLHeli_32 said that the ESC was 8bit, not 32bit.
Downloaded THIS ONE
Connected to ESC. Read ESC for existing parameters. Changed Startup Volume from 150 (or somewhere around there) to 10. Wrote new parameters to ESC. ESC DIDN’T GO BANG.
Happy with this I opened the ROV electronics enclosure and did the same process with all 8 ESCs.
I now have a ROV that doesn’t sound like an electronic screaming banshee on startup.
Many thanks @tony-white @EliotBR and most of all @rick for sending us down this path.